There has been proposed an image forming apparatus, such as a laser printer, having a photosensitive drum carrying a toner image and a transfer roller attracting the toner image, both of which remain in contact with each other. When a sheet passes between the photosensitive drum and the transfer roller, the toner image moves to a transfer roller and is transferred to the sheet so as to produce an image. Incidentally, such an image forming apparatus has a developing cartridge for feeding toner to the photosensitive drum.
JP-A-2006-98770 discloses a related art developing cartridge including a box-shaped case having an oblong opening; a developing roller that is exposed through the opening of the case and that feeds toner to the photosensitive drum; a layer thickness regulation blade that makes a slidable contact with the developing roller to regulate the thickness of toner on the developing roller to a predetermined thickness; and a seal member that stops up a gap between the developing roller, the layer thickness regulation blade, and an edge of the opening of the case.
The related art developing cartridge is divided into an upper frame, which has a plate-shape and is provided at the layer thickness regulation blade side with reference to the developing roller, and a lower frame having the edge of the opening. A part of the edge of the opening that nips the seal member between the edge and the layer thickness regulation blade is formed as a beam-like portion that extends across right and left sidewalls of the lower frame. In this structure, a front end portion of the upper frame (i.e., an opening-side portion of the upper frame) is supported by the beam-like portion, and right and left end portions and a rear end portion of the upper frame are supported by the right and left walls and a rear wall of the lower frame.
However, the related art developing cartridge has some disadvantages. For example, when the lower frame has the beam-like portion, die cutting must be performed from a front opening of the lower frame to the front and from an upper opening of the lower frame upward. Thus, production method of the related art developing cartridge becomes complicated.